The development of tools capable of analyzing, controlling, and forming materials on the nanoscale has led to an interest in creating new structures and devices at these dimensions. This ability to control matter on an atomic and molecular scale is the basis for the rapidly developing field of nanotechnology. As the size of a system continually decreases, its physical, optical, and electrical properties may differ significantly from, and even be superior to, those observed at the macroscale. These differences occur primarily due to quantum size effects and a significant increase in the surface-to-volume ratio as a consequence of the reduction to nanoscale dimensions. Moreover, the properties of such materials may differ significantly based on whether the material exists in an equilibrium state or a metastable state. Indeed, metastable solids have long been used in technology to access a much wider range of materials properties than those of the limited set of equilibrium phases. These changes strongly influence a number of materials properties which include the electronic, optical, mechanical, thermal, and chemical properties of the material itself.
From among the various nanomaterials that have emerged, zero- and one-dimensional nanostructures have attracted significant interest due to the potential for developing materials and devices with unique electrical and optical properties. The metastablity of such materials has also attracted significant interest due to the potential for harnessing specific functional properties of metastable phases. Potential applications include quantum devices utilizing carrier confinement, photovoltaic cells, sensors, optical switches, electronic devices such as field-effect transistors (FETs), p-n junctions, and bipolar junction transistors, and for metastable phases, potential for superconductivity. One of the means to the realization of practical applications utilizing these materials is the ability to form nanostructures with controlled dimensions, structures, phases and compositions. Such control would enable tailoring of the electrical, optical, and mechanical properties to a particular application. Another aspect is the ability to form these nanostructures on a large scale at minimal cost.
The synthesis of zero-dimensional (0-D) nanostructures or quantum dots has been well-studied with a plurality of methods having been developed to form 0-D nanostructures in designated locations with well-controlled dimensions using a variety of materials. One-D nanostructures are often defined as structures with lateral dimensions falling within the range of 1 nm to 1 μm, but more usually to those in the range of 1 nm to 100 nm and longitudinal dimensions extending essentially indefinitely. Thus, while 0-D structures are generally discrete particles which are dimensionally symmetric, the formation of 1-D nanostructures requires that growth be constrained in two dimensions, yet extended along a third.
Top-down approaches which have been used to fabricate a variety of 1-D nanostructures from different materials include lithographic techniques such as electron-beam or focused-ion-beam etching, scanning probe patterning, and X-ray or extreme ultraviolet lithography. However, these techniques are not feasible for efficient large-scale manufacturing of 1-D nanostructures at low cost. Consequently significant effort has been focused on developing alternative methods to efficiently produce a large number of 1-D nanostructures from a wide range of materials. A majority of these have focused on bottom-up approaches in which 1-D growth strategies have been formulated based upon chemical synthesis routes. A review of these strategies is provided, for example, by Y. Xia in “One-Dimensional Nanostructures: Synthesis, Characterization, and Applications,” Adv. Mater. 15, 353 (2003) the entire contents of which is incorporated by reference as if fully set forth in this specification.
Growth strategies which have been employed to form 1-D structures include, for example:                a) exploiting the anisotropic crystal structure of select materials;        b) using a liquid-solid interface to direct growth of a crystal;        c) forming a 1-D template to constrain growth to a specific direction;        d) controlling supersaturation of the gas phase to modify the growth mode;        e) use of capping reagents to kinetically control the growth rates on different crystal facets;        f) self-assembly of multiple 0-D nanostructures; and        g) direct size reduction of 1-D microstructures.From among the various approaches followed, one of the most extensively investigated approaches involves the vapor-phase synthesis of nano structures such as 1-D whiskers, nanorods, and nanowires (NW).        
Vapor-phase synthesis primarily involves manipulating the degree of supersaturation of the gas phase to control the growth mode. One approach, termed vapor-liquid-solid (VLS) growth, has been used to successfully produce high-quality single-crystal NWs in significant quantities. The VLS process was used by Wagner and coworkers in the 1960's to form Si “whiskers” as described in “Vapor-Liquid-Solid Mechanism of Single Crystal Growth,” Appl. Phys. Lett. 4, 89 (1964), which is incorporated by reference herein in its entirety. Central to the VLS process is the use of a catalyst comprising a metal or metal alloy to direct NW growth; in the case of Wagner's Si whiskers the catalyst was a gold-silicon (Au—Si) alloy. The catalyst is initially dispersed across the surface of a substrate as suitably-sized nanoparticles which transform to the liquid alloy phase upon heating and supplying of semiconductor material. The liquid alloy nanoparticles absorb atoms from the vapor phase, facilitating the nucleation of crystal seeds at the liquid-substrate interface from which NW growth can occur. The material constituting the growing NW and the nanoparticle form a liquid-phase binary alloy drop whose interface with the growing wire represents the NW growth front. Under steady-state growth conditions, adsorption on the drop surface maintains a concentration gradient of the NW component of the liquid binary alloy, which is counteracted by a diffusion current through the drop. This liquid phase transport, in turn, causes a small supersaturation driving the incorporation of new material at the drop-NW interface to continually extend the wire. This growth process is described, for example, by E. I. Givargizov in “Fundamental Aspects of VLS Growth,” J. Cryst. Growth 31, 20 (1975) which is incorporated by reference in its entirety as if fully set forth in this specification. Givargizov used Pd, Ni, and Pt, in addition to Au, as liquid-forming agents (growth catalysts) for Si.
The VLS process has now been used to synthesize NWs from a wide variety of inorganic materials including, for example, group IV (Si and Ge), IV-IV (SiC), III-V (GaN, GaAs, GaP, InP, and InAs), II-VI (ZnS, ZnSe, and CdSe), and IV-VI (PbSe, PbS) semiconductors as well as oxides such as ZnO, MgO, and SiO2. This has been demonstrated, for example, by X. Duan, et al. in “General Synthesis of Compound Semiconductor Nanowires,” Adv. Mater. 4, 298 (2000) and “InP Nanowires as Building Blocks for Nanoscale Electronic and Optoelectronic Devices,” Nature 409, 66 (2001), as well as by T. I. Kamins, et al. in “Growth and Structure of Chemically Vapor Deposited Ge Nanowires on Si Substrates,” Nano Lett. 4, 503 (2004) (“Kamins 2004”), each of which is incorporated by reference in its entirety as if fully set forth in this specification. Note that the VLS growth of compounds such as GaAs and GaP need not require the use of an exotic catalyst; they may instead be grown via the formation of a liquid in which one or the other element is enriched compared to the equilibrium solid composition.
Despite its widespread use, key aspects of the VLS growth process remain poorly understood. Consequently, little is known about how to control nanowire dimensions during growth primarily due to the fact that, in contrast to studies on conventional thin film deposition, measurements with nanometer spatial resolution are needed to analyze the mechanisms of NW growth. While progress has been made, there remains a need to develop methods for fabricating a large number of high-quality nanowires with well-controlled dimensions, chemical compositions, surface morphologies, and nanostructures. It is therefore necessary to formulate strategies to control not only the nucleation, growth mode, average diameter, and length of individual nanowires, but also the local site-specific diameter during growth itself.